1. Field of the Invention
The present invention relates to a method for acquiring the physical information of the vehicle driven on the road, and in particular to a method for detecting a vehicle type, a vehicle speed and width of a detecting area by a vehicle radar sensor.
2. Description of the Related Art
In the prior art, radar technology is ordinarily applied in some professional fields such as car speed detecting, meteorology, and aviation tracking. The radar device of the radar technology comprises a radio wave transmitter that transmits radio waves to trace an object, and a radio wave receiver that gathers reflective radio waves. According to the Doppler Effect, the frequency of the radio wave will become higher than the original frequency when the object approaches the radar device, and the frequency of the radio wave will become lower than the original frequency when the object moves away from the radar device. Therefore, the frequency variations of the reflective signal is analyzed to acquire the object's speed; in other word, when a vehicle moves at a high enough speed to generate the Doppler Effect, the reflective radio wave from the vehicle will generate the Doppler shift. The Doppler frequency versus time variations of the reflective radio wave is recorded and the relative speed of the object and the radar can be computed.
Refer to FIG. 10. It is a simple system that a radar device detects the vehicle speed. The radar device 34 transmits a radio wave that is distributed over a specific area. The vehicle is viewed as an isotropic point target 38 entering into the specific area with a constant speed and reflecting a reflective signal. The radar device catches the reflective signal and utilizes the signal effect of the Doppler Effect to calculate the point speed. Because the equation that describes the relation of the speed of the isotropic point target 38 and the Doppler shift is
  DopShift  =                    2        ⁢        v            λ        ⁢          sin      ⁡              (        ϕ        )            
Whereas, φ is an angle 40 shown in FIG. 10 and the angle is changed with the moving point 38. If φ is very small, sin(φ) is closed to the value of φ which is described as the following equation:
  ϕ  =            arctan      ⁡              (                  vt          R                )              ≈          vt      R      
Whereas, t is the time of the point 38 arriving at the vertical location to the radar device, and R is the distance that the point 38 moves at the vertical location to the radar device.
Therefore,
      DopShift    ≈                  2        ⁢                  v          2                ⁢        t                    λ        ⁢                                  ⁢        R                        i      .      e      .                          ⁢              DopShift        t              ≈                  2        ⁢                  v          2                            λ        ⁢                                  ⁢        R            
From the above mentioned equations, it can be known that, if point 38 moves at a constant speed, the Doppler shift versus time is expected to be a linear distribution, as shown in FIG. 11. According to the above technology of the prior art, the frequency of the reflective radar signal is gathered and analyzed to obtain a distribution diagram representing Doppler frequency versus time variations of the object, and then the speed of the object, i.e. the vehicle speed can be known. But, some information, like the vehicle length and the width of the detecting area, cannot be further acquired in the prior art.
Moreover, the above mentioned technology is only useful in a specific situation that the object is far away from the radar device, so it can be viewed as an isotropic point target 38, so it can be viewed as an isotropic reflective object. The vehicle to be detected by the radar device is in a short distance of about tens of meters. However, the vehicle is not an isotropic reflective object, so the distribution is not a straight line 44 in the diagram of the Doppler frequency versus time distribution, but it is a consecutive motion diagram, a parallelogram 42, as shown in the diagram (Refer to FIG. 12). Because the object has volume in the three dimension space, the reflective radio waves are reflected by the whole body, thus the reflective signals form the parallelogram 42 of the distribution. The prior art never further studies any information based on the distribution, and is also not aware of the physical meaning of it. The prior art always searches a line 44 based on the parallelogram of the distribution but the line 44 is not a correct bevel 46 of the parallelogram, as shown in FIG. 12. If the vehicle speed is acquired according to the wrong line, the inaccuracy of the speed could be not stable. In the view of these above mentioned shortcomings of the prior arts, the present invention provides a method to detect the more accurate and stable speed by analyzing the consecutive motion diagram: a parallelogram to acquire an exact vehicle length and an exact width of the detecting area, for the application of the person skilled in the art.